1. Field of the Invention
The present invention relates to a liquid crystal display device substantially free from "cross-talk" and exhibiting a high brightness.
2. Description of the Related Arts
It is known that liquid crystal display (LCD) devices can be directly operated by using an IC device under a low drive voltage at a low power consumption, and can be easily constructed as a compact and thin device. In particular, TN type LCD devices are advantageous in that they can be operated under a low voltage at a low power consumption, and thus are widely utilized in various fields; for example, watches and desk-type electronic calculators.
The recent growing popularity of word processors, personal computers and other data processing devices has led to a demand for portable, small and thin devices of this kind, and to this end, LCD devices are replacing the cathode ray tube (CRT) devices as the display element in those machines. With the LCD device, it is necessary to reproduce the images of Chinese characters on the screens of those information-processing machines, since a great number of picture elements must be used in the LCD device in comparison with the number of the picture elements in the display devices for watches and desk calculators. Also, the LCD device can be operated by a simple matrix drive circuit in which a number of picture element electrodes are connected to signal lines in an X-Y matrix form. In this type of simple matrix drive system, the electrodes corresponding to the picture elements are not independent from each other, and thus when a predetermined voltage is applied to one picture element electrode, the voltage is also applied to the adjacent picture element electrodes, whereby the adjacent picture element electrodes are partly operated, i.e., an undesirable "cross-talk" occurs between the adjacent picture element electrodes.
To eliminate this cross-talk, it is known to utilize a non-linear element consisting of a diode, for example, a metal-insulator-metal diode (MIM), thin film transistor (TFT), or varistor for each picture element electrode, but it is very difficult to provide and arrange a large number, for example, several thousands to several hundreds of thousands, of diodes or thin film transistors corresponding to a large number of picture elements, all of which must have uniform properties and be free from defects, and thus there is an urgent need for the development of non-linear elements which can be easily provided with a uniform quality and will allow the use of an LCD device with a large display area.
In connection with the above, a new type of liquid crystal material usable for a display in a large area has been developed, through a new technology involving a polymer dispersed liquid crystal material, for example, an NCAP (nematic curvilinear aligned phase) type liquid crystal material containing a number of small liquid crystal particles dispersed in a polymeric matrix, or a liquid crystal composite membrane having a continuous phase liquid crystal contained in a polymeric matrix in the network form.
With this technology, it is now possible to easily control the thickness of the liquid crystal material layer in the display device, and therefore, the polymer-dispersed liquid crystal material can be formed as a layer having a large surface area, exhibits a very quick response, and allows the resultant display device to be given a wide angle of view without the use of a polarizer plate, and thus can be advantageously utilized to provide an LCD device having a large display surface area.
Nevertheless, the polymer-dispersed liquid crystal material is disadvantageous in that this liquid crystal material requires a drive voltage of from several tens to several hundreds of volts, which is remarkably higher than that of the TN type liquid crystal material, e.g., 5 volts or less. Therefore, a new-type of non-linear element which has a higher voltage resistance than that of conventional non-linear elements, for example, thin film elements, and can be driven at a low drive voltage, must be used for the polymer dispersed LCD device.
The inventors of the present invention attempted to provide a new-type of liquid crystal display device which has the polymer-dispersed liquid crystal material layer and in which a membrane consisting essentially of varistor particles is utilized as a non-linear element. This type of LCD device can display clear images or pictures without the occurrence of cross-talk, even where the device is operated at a high drive voltage.
Nevertheless, this type of display device is disadvantageous in that, when the picture element electrodes are very small, the proportion of the areas of the varistor layers through which the picture element electrodes are connected to the corresponding signal lines with respect to the total area of the display device becomes large, and therefore, the total display area in which the visible images are generated becomes small. If the areas of the varistor layers are reduced to avoid the reduction of the display area, the varistor voltages of the resultant varistor layers become variable (not constant), and thus it becomes difficult to obtain an even display of images. Also, due to the reduction in the areas of the varistor layers, it becomes impossible to flow a sufficient amount of electric current through the varistor layers, and thus the brightness (luminance) of the display face of the device is reduced. Namely, the brightness (luminance) is reduced by an increase in the duty ratio of the display device.